This summer, location-based social network Foursquare moved its San Francisco office from a small space it shared with The Chronicle to a big new office in SoMa.

In May, the service reported 7.5 million users. Today, it has more than 15 million. The company has grown along with it, going from seven San Francisco employees to 20 during that period.

Despite that growth, the company remains a cipher to many Americans. A report from Forrester Research this month said that only 5 percent of U.S. adults had used a location-based service in the past month, up just one percentage point from the previous year.

That poses a challenge to Dennis Crowley, who co-founded Foursquare in 2009 with Naveen Selvadurai. The service lets users win virtual badges and other rewards by "checking in" via phone to the places they visit. Increasingly, the company is using check-in data to provide quality recommendations for places to explore. And more than 600,000 merchants are using it to offer deals to people who check in.

As with Dodgeball, an early location-based service that he sold to Google, Crowley said the goal of the service is "make the real world more usable." But to do that, users have to embrace its recommendations.

Recently, we sat down with the 35-year-old co-founder to talk about check-ins, big data and where the service is heading next.

Q:So Forrester this month reported that 5 percent of U.S. adults are using location-based services this year. You guys have gained a ton of users, but it seems like this is not something that is gaining mass popularity. Is it going slower than you expected?

A: I've been doing location and mobile stuff for a while - for more than 10 years now. So the fact that there are 15 million people on Foursquare is a big deal. We get more users in half a day than Dodgeball had in its entire existence. So, I don't really pay much attention to those studies.

If you look at the Forrester studies for Twitter three years ago, it's like, "This isn't interesting. There's 1 percent of people tweeting what they had for lunch." I'm sure they nailed their research story, but it didn't turn out to be true.

We've been in this long enough to know we're on to something very good - actually, we're on to something great. We timed it much better this time. We just keep doing what we're doing and make sure the stuff works the way we think it should work.

Q:I've found that my own group of friends tends to check in less often than they used to. Recently you've spoken about wanting to de-emphasize check-ins. Is there a broader trend there?

A: It's true of any social service - you always see some drop, so it's kind of natural. What we're experimenting with is different ways to pull those users back in. Do you tag someone in a check-in, does that pull them back in? Do you send them an e-mail newsletter, does that pull them back in? If they see leveled badges, does that pull them back in? Overall, it's been good.

The engagement levels have been good. The most interesting thing is what we're starting to see with people who open the app but don't check in. At first we said, is something broken? What's going on? And then you realize that they're benefiting from the ecosystem. They use (Foursquare's "Explore" feature) to see what their friends are doing, they look at the tips at a restaurant.

So when people say, you're all about the check-in - well, we are; check-ins give us a ton of data points. But it's the same thing as Twitter: Not everyone on Twitter tweets. There are people who don't even know how to send tweets that are getting a lot of value out of Twitter. We're mimicking that pattern.

Q:You have a billion check-ins now in your database. You're becoming a big-data company. What can you do with all that information?

A: I'm always surprised when people say Foursquare is pivoting to recommendations; it's not pivoting. This has been the master plan the whole time.

We knew this is what we wanted to get to: the thing that can tell you where you want to go, based upon what you've done and what your friends are doing. But in order for that to even work, you have to have a lot of check-ins.

How do you get a lot of check-ins? You've got to make something that people want to check in with. And that involves points and badges and tips. We built all that stuff knowing that if we did a good job, we would end up where we are right now.

Q:Recently you've started encouraging people to create lists of places they'd like to visit. Getting people to use lists has been a tough nut to crack for companies like Facebook and Twitter - it hasn't been a mainstream use case. Are you seeing lists catch on with Foursquare users?

A: Remember, our lists are different: Our lists aren't lists of people, they're lists of places. People make lists of places all the time. I'm going to Chicago, what should I do? Let me jot down five ideas.

That's what it's meant for. Are you going to be in New York? Let me put together a list and send it to your phone; it will buzz when you land. We went down the list-of-users thing with Dodgeball. Because users asked for it - "I don't want to broadcast to everyone, I want to broadcast to a subset!" And we said OK, we'll build that. And no one used it.

I just don't think people like making lists of their friends. It's not how we think about our friends. It's a pain to make a wedding list, a holiday party list. Nobody likes to do that, to exclude people. But if I make a list of the 10 best things that you should do in Paris when you go back, that's a fun list for me to make. And I think that's why you're seeing some adoption of lists on Foursquare.

Q:You guys have gained more traction than most social networks, but I still can't help but wonder what Foursquare would feel like if I had 300 friends using the service instead of just a few dozen.

A: It will happen. I really think our biggest roadblock is the idea that you have to be thinking about it to use it. It's also the roadblock for things like Twitter. And then you just get to a scale where it just starts working for people.

We're getting bigger. We haven't hit that tipping point yet. When it will happen? I'm not really sure. But in my mind, I know what the product looks like when that happen. There are things that need to change, there's a little more polish. And I think we're getting pretty close to it.

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